The present invention relates to a digital radio paging communication system in which a base station (or a transmitter) selectively calls a plurality of receivers, and more particularly to a digital radio paging communication system having receivers capable of battery saving (or low power drain).
In the nation-wide paging system proposed by the British Post Office, a base station transmits POCSAG signals or codes, each consisting of a preamble signal and a plurality of batches of code signals. (Each batch further comprises a synchronization signal and eight groups of calling signals).
A paging receiver, on the other hand, temporarily discontinues its battery saving operation in response to the preamble signal and achieves word synchronization in accordance with the synchronization signal. Then, it cuts off its power supply to its circuits, except for its timing circuit. The receiver restores its power supply to the circuits in the position of its own code group to detect calling signals and then again cuts off its power supply upon completion of its own code group. Further, upon detection of the synchronization signal in the next signal batch, the receiver repeats the above-described procedure and, if no synchronization signal is detected or if an idle signal is detected in its own code group, it returns to the battery saving operation.
However, such a receiver, as will be further described hereinafter, embodies no consideration for the period of time required, in building up the power supply in its own code group. The time period begins with the initiation of the power supply operation and continues until the receiver's stable signal receiving operation occurs. The receiver detects each group of signals only by bit synchronization, which is established with the preamble and synchronization signals. However, the receiver does not receive any signal for renewing bit synchronization within its own group or between its own group and the immediately preceding group. The result is that the calling signals of each group may be erroneously received under electric field fluctuations, such as fading. It further involves the problem that, if an uneven or biased pattern prevails, such as the consecutive occurrences of a logic "1" or "0" in digital signals, the reference voltage in a waveform shaping circuit in the receiver, as will be described hereunder, becomes biased. Consequently, correct data reproduction takes time.
Meanwhile, in the paging system, such as the one disclosed in the U.S. Pat. No. 4,194,153 assigned to the same assignee as the present invention, the base station transmits digital burst signals consisting of a preamble signal, a plurality of calling signals and an end mark signal. The receiver, on the other hand, discontinues its battery saving operation in response to the preamble signal, detects the ensuing calling signals and, when it detects the end mark signal, returns to the battery saving operation. In this system, however, once the receiver discontinues its battery saving operation in response to the preamble signal, it continues to consume power until it detects the end mark signal, even in the absence of its own calling signals. Accordingly, especially where there is a long block of calling signals, it needs improvement with respect to efficient use of power supply.